Preheating strategies and exergy analysis for hydrogen direct reduced iron systems

In response to the high carbon emissions of the steel industry, hydrogen direct reduction technology has emerged as a promising pathway for low-carbon transformation. A thermodynamic model that integrates direct reduction, waste heat recovery, water electrolysis, and material preheating has been proposed and investigated to assess the impact of hydrogen and iron ore preheating temperatures on system exergy efficiency in a reduction temperature range of 700–1150 °C. The results indicate that the minimum hydrogen-to-iron molar ratio (Nmin) exhibits a strong linear decrease from 20.56 to 12.81 as the reduction temperature increases from 700 to 1150 °C. When the reduction system operates at Nmin, if only the heat required for reduction is provided by preheating hydrogen, the hydrogen needs to be preheated to 946 °C to 1632 °C. The preheating temperature of hydrogen can be reduced by simultaneously preheating hydrogen and iron ore, or by increasing the hydrogen-iron molar ratio (N) or directly supplementing heat such as electric heating. However, increasing N generally reduces the hydrogen utilization rate. Exergy analysis indicates that the system reaches maximum exergy efficiency at the corresponding Nmin for each reduction temperature, with a global maximum of 68.18% achieved at 1000 °C under Nmin = 14.10. Under this condition, as the H2 preheating temperature varies from 859 °C to 1409 °C, the exergy loss share of the reduction subsystem ranges from 5.8% to 13.9%; when H2 is preheated to 1050 °C, the exergy loss of the reduction subsystem reaches a minimum of 4.9%.